-One or more users currently use a mailbox store on this server. These users must be moved to a mailbox store on a different server or be mail disabled before uninstalling this server."

Resolution

This issue occurs because the Setup program detects that a mail-enabled user is connected to this mailbox store. There may also be system mailboxes or other hidden mailboxes that are not visible in Exchange System Manager or in Active Directory Users and Computers.

Determine whether one or more users have the affected server listed as their Exchange home server and then move the mailbox or remove the Exchange attributes of the user.

You do not have to consider the following mailboxes:
- SystemMailbox
- Microsoft DSA
- SMTP
- Microsoft System Attendant

A combination of all 3 methods below may be required to find all affected accounts.

The server that appears in this column is the same value that appears in the msExchHomeServerName attribute.

Method 2 (Active Directory Users and Computers):
- Right-click the domain container, and then click Find
- Select the Advanced tab
- Select User from the Field button
- From the list of attributes displayed, choose Exchange Home Server
- Set the Condition field to Ends With
- Enter the Exchange server name into the Value field
- Click Add
- Click the Find

Error message when you try to remove Exchange 2000 Server from a mailbox server that no longer hosts mailboxes: "One or more users currently use a mailbox store on this server"
http://support.microsoft.com/kb/924170